Recent developments in laser technology have made practical the commercial production of laser-based consumer electronic products. One such product is a video playback unit which uses a focused laser beam to optically retrieve a previously recorded video signal from a spiral recording track on a rapidly rotating reflective optical recording disc. Since no stylus is required to mechanically engage a groove in the recording disc, the optical disc is subject to virtually no wear during playback. Additionally, since the disc includes a protective coating, the outer surface of which is substantially removed from the plane of focus of the laser beam, the quality of the recovered signal is relatively unaffected by the presence of dust, fingerprints or scratches on the disc surface.
While these attributes make optical laser discs an ideal medium for conveying pre-recorded television and audio signals, there are technical problems which must be overcome in developing a practical playback unit. One of these problems involves accurately following the spiral recording track with a focused laser beam during playback. Since a typical recording track is approximately 0.0004 millimeters in width and moves outward a distance of about 0.0016 millimeter per revolution, it will be appreciated that considerable precision is required in following such a track with a laser beam.
During playback, a standard laser disc is rotated by means of a turntable at standard speeds of either 1800 or 1500 r.p.m. A movable laser beam deflection carriage, containing optical elements which direct and focus the laser beam, moves radially outward from the center of the turntable beneath the lower surface of the rotating disc. The motion of the carriage matches that of the relative radial motion of the recording track so that the laser beam approximately follows the path of the track. Because it is difficult if not impossible to achieve the required precision by purely mechanical means, many systems, such as those described in the U.S. Pat. Nos. 4,236,232, 4,359,635 and Re. 29,963, have been developed which employ an electrical feedback signal to minutely control the instantaneous position of the focused laser beam in order to precisely follow the recording track. While such electromechanical systems are effective, it is still necessary to reduce the mechanical discrepancies between motion of the carriage and of the track to a minimum in order to secure best performance. Accordingly, it has been an object of designers of such playback units to minimize imprecision in the mechanical structure of optical playback units.
One common source of mechanical imprecision is recording track eccentricity produced by reason of the recording disc not being precisely aligned with the axis of the rotating turntable. Accordingly, manufacturers of optical recording discs must carefully control the location and diameter of the spindle aperture in the disc, making these as close as possible to those of the turntable hub.
However, the spindle aperture must not be so small as to interfere with normal installation and removal of the disc in the face of normal manufacturing tolerances. Consequently, the spindle aperture is sized slightly larger than the hub. Some means for clamping or otherwise securing the disc to the turntable is then required to assure that no relative motion of the disc and turntable occurs during operation of the playback unit.
The present invention overcomes the problem of disc misalignment by providing a tapered hub assembly which includes centrifugally-actuated clamp members which operate when the disc and turntable are rotating to automatically clamp the disc on the hub. Slight variations in the diameter of the spindle aperture are automatically compensated for by the tapered surface hub, which centers the disc relative to the axis of the turntable regardless of the size of the spindle aperture.